1. Field of the Invention
The present invention relates to an apparatus for and a method of measuring thermal stresses attributed to the heat of hydration generated from concrete structures, which can be used in the laboratory instead of conventional analytical techniques or experiments performed on actual concrete structure.
2. Description of the Prior Art
Recently, active research has been directed to the development of cement which is free from generating heat upon hydration because the heat of hydration causes significant problems in constructing massive concrete structures or effecting high strength concretes. A concrete structure undergoes a certain thermal stress caused by the heat of hydration of cement, according to confining conditions given to its inner and outer portions. In extreme cases, the thermal stress may cause cracks in concrete structures to deleteriously affect the utility, water tightness and durability thereof.
It is, therefore, very important to quantitatively examine the influence of the heat of hydration on the degree of thermal stress and the occurrence of cracks. Generally, thermal stresses generated in concrete structures are inferred analytically or experimentally.
For analytical inference, finite element methods are usually employed, in which a variety of construction conditions can be effectively modeled. The experimental method to infer the thermal stress is further broken down into two manners: one is to apply equipments and gauges directly to actual or model structures; and the other is to utilize testing apparatuses for measuring thermal stresses, which are manufactured for laboratory use.
Because it makes assumptions on the behavior of all parameters, the analytical method is apt to give inaccurate results for early-age concrete, whose physical properties cannot be clearly determined. This problem is also not readily solved by the experimental method using actual or model structures. Additionally, the experimental method suffers from the disadvantage of requiring a large expenditure because an experiment must be carried out during the construction.
The testing apparatuses, most of which are manufactured in Germany or Japan, are very expensive and have difficulty in modeling actual thermal stresses generated in concrete structures. Additionally, the apparatuses cannot detect the changes in thermal stress of the structure subjected to internal and external confinements. The change in stress generated from the inside of a structure, which is subjected to internal and external confinements, as shown in FIG. 1a, has a tendency to be opposite to that of the change in the stress generated at the circumferential portion of the structure, which is subjected to internal confinement, as shown in FIG. 1b. Those apparatuses cannot effectively reflect the tendency. Also, the testing apparatuses are economically unfavorable in that additional devices are required to describe the thermal change at some sites of the structure.
One of prior arts is described by Tazawa and Iida (Transaction of the Japan Concrete Institute, Vol. 5, E. Tazawa and K. Iida, pp. 119-126). They suggested a thermal crack testing apparatus which consists of 4 stainless steel pipes and plates fixed to the opposite ends of the pipes via nuts. A strain gauge is equipped on the pipes through which water is circulated at a predetermined temperature. A laboratory in which a sample is placed is suitably controlled for temperature to embody the thermal hysteresis obtained by temperature analysis. A confinement degree is determined according to the change in the stiffness of the pipes and the temperature of the circulating water.
After performing an experiment under semi-adiabatic conditions, Breitenbucher (Material and Structures, Vol. 23, R. Breitenbucher, pp. 172-177) introduced a cracking frame which is designed to arbitrarily control the temperature of the concrete by embedding a copper pipe in a form.
A temperature stress testing machine was disclosed in Proceedings of the International RILEM Symposium, 1994, R. Springenschmid and R. Breitenbucher, pp. 137-144, which is identical in overall mechanical design and shape to the cracking frame, but different in that thermal stress is measured through a load cell equipped on one cross-head portion while concrete dislocation is controlled by use of a step motor.
The conventional techniques described above, however, differ from the invention in technical constitution.
It is an object of the present invention to provide a testing apparatus which can conveniently measure thermal stresses generated in concrete structures indoors.
It is another object of the present invention to provide a testing apparatus for measuring thermal stresses of concrete structures, which can account for the effect of the internal and external confinements in concrete structures by use of a material different in coefficient of thermal expansion from concrete.
It is a further object of the present invention to provide a method of measuring thermal stresses of concrete structures, which can take accurate account of the effect of the temperature on the thermal stresses by testing in a temperature and humidity chamber with a temperature hysteresis obtained in a certain portion of the concrete structures.